[Paper Review] Comment on "Spontaneous liquid-liquid phase separation of water" by T. Yagasaki, M. Matsumoto and H. Tanaka, Phys. Rev. E 89, 020301 (2014)

David T. Limmer, David Chandler|arXiv (Cornell University)|Jun 10, 2014

Spacecraft and Cryogenic Technologies3 citations

TL;DR

This comment challenges the interpretation of Yagasaki et al.'s molecular dynamics simulation of supercooled water, arguing that observed density fluctuations and domain coarsening do not indicate two distinct liquid phases or criticality. Instead, the results are consistent with well-established coarsening dynamics in metastable supercooled water, with no evidence for a liquid-liquid critical point or finite surface tension between phases.

ABSTRACT

Yagasaki et al. present results from a molecular dynamics trajectory illustrating coarsening of ice, which they interpret as evidence of transient coexistence between two distinct supercooled phases of liquid water. We point out that neither two distinct liquids nor criticality are demonstrated in this simulation study. Instead, the illustrated trajectory is consistent with coarsening behaviors analyzed and predicted in earlier works by others.

Motivation & Objective

• To clarify that the simulation results by Yagasaki et al. do not demonstrate two distinct liquid phases of water.
• To challenge the interpretation that the observed density fluctuations indicate a liquid-liquid critical point.
• To argue that the system's behavior is consistent with non-equilibrium coarsening dynamics rather than phase coexistence.
• To emphasize that finite surface tension—required for distinct phases—is not supported by the data or prior theoretical analysis.
• To reaffirm that the observed phenomena align with established models of ice nucleation and coarsening in supercooled ST2 water.

Proposed method

• Analysis of a 1 μs molecular dynamics trajectory of supercooled ST2 water under constant volume and particle number.
• Comparison of observed domain fluctuations and interfacial dynamics with prior theoretical predictions from Limmer et al. (2013) and Molinero et al.
• Use of time-scale separation analysis to distinguish early-stage fluctuations from late-stage long-range ordering.
• Evaluation of interfacial properties, including surface tension, to test for phase coexistence.
• Examination of domain size distribution and interfacial fluctuations to rule out critical behavior.
• Use of visualizations and videos from Yagasaki et al. to illustrate dynamic evolution of domains and interface morphology.

Experimental results

Research questions

• RQ1 Do the observed density fluctuations in the simulation represent coexistence of two distinct liquid phases of water?
• RQ2 Is there evidence for a liquid-liquid critical point in the ST2 water model based on the simulation?
• RQ3 Can the observed domain coarsening be explained by non-equilibrium dynamics rather than phase separation?
• RQ4 What is the role of surface tension in determining whether two liquid phases can coexist?
• RQ5 Are the time scales and amplitude of fluctuations consistent with established coarsening theories?

Key findings

• The simulation shows no evidence of a finite surface tension between the two domains, which would be required for distinct liquid phases.
• The observed large interfacial fluctuations are inconsistent with a critical point, which would require many domains of varying sizes.
• The time scales of density fluctuations (tens of nanoseconds) and long-range ordering (after 1 μs) match predictions from prior coarsening models.
• The final state features a sharp, stationary interface between ice and liquid, consistent with phase separation in a metastable system.
• The system evolves through non-equilibrium dynamics, with ice nucleating from the lower-density domain, as predicted by earlier work.
• The results are fully consistent with coarsening in a metastable supercooled liquid, not with coexistence of two distinct liquid phases.